The present invention relates to cement clinker and cement containing the cement clinker, and more particularly to an improvement in cement for use in concrete of high strength and high flowability, mass concrete, shrinkage compensating concrete or concrete of high resistibility which are employed in the field of engineering and architecture or as building material through incremental launching method or wet formation of cement slurry.
Accompanying promotion of rationalization and energy saving in the field of construction and working techniques in these years, needs for cement of large variety and complexity are increasing, and such cement is being applied to concrete of high strength and high flowability for use in high-rise reinforced concrete buildings or buildings of concrete filled steel tubular column (CFT). Such cement is also applied to so-called mass concrete for use in concrete dams or concrete buildings of large sized members.
In order to obtain concrete of high strength, the water cement ratio is required to be decreased which results in an increase in viscosity of the concrete and a loss in flowability which in turn leads to a drawback in that the workability may be degraded, for instance, the pumpability may be degraded.
Further, a decrease in water cement ratio results in an increase in the amount of cement and thus causes an increase in calorific value at the time of hydration reaction, whereby a drawback is presented in that the strength development of the structure decreases by the effect of thermal hysteresis of high temperature.
Due to these drawbacks, it is generally the case that normal portland cement can only be used at a specified concrete strength of not more than 45 N/mm2 when employed for constructing high-rise reinforced concrete buildings or CFT buildings. In other words, in order to achieve a specified concrete strength of approximately 60 N/mm2, the water cement ratio needs to be decreased to approximately 30%, which, however, results in a remarkably high viscosity of concrete and a loss in flowability so that the pumpability may be degraded.
Further, when using blended cement, the viscosity of concrete becomes lower than that of normal portland cement when both are prepared employing the same water cement ratio. However, since the strength development of the blended cement is inferior to that of normal portland cement, to obtain equivalent strength, it is required to set a lower water cement ratio for the blended cement than that of normal portland cement. This, in turn, results in an increase in viscosity of concrete with no sufficient flowability being maintained. Consequently, such blended cement can be used to obtain only a level of strength equivalent to that of normal portland cement.
In contrast to that, when using low-heat portland cement, the viscosity of concrete becomes lower than that of normal portland cement when both are prepared employing the same water cement ratio, and their strength developments are equal to each other. Thus, it is capable to pump the low-heat portland cement by a pump in a strength region being higher than that of normal portland cement, and more particularly, at a specified concrete strength of up to approximately 60 N/mm2.
The use of low-heat portland cement is desirable also in view of the decrease in the calorific value of hydration.
However, even by employing low-heat portland cement, the viscosity can still not be sufficiently decreased, and when the specified concrete strength is set to be as high as approximately 80 N/mm2 (approximately 25% in a water cement ratio), its viscosity is remarkably increased. Therefore, it can not be pumped by using a general pump and types of pumps and conditions for the pumping are limited.
Low-heat portland cement presents an additional drawback in that its initial strength is lower than those of normal portland cement or blast-furnace slag cement. In order to solve this drawback, measures are taken in that the fineness as well as the amount of contained SO3 are increased for the purpose of improving the initial strength.
However, while an increase in the fineness and the amount of SO3 contributes to improvements in strength, it simultaneously results in a drawback of increasing the calorific value of hydration and of decreasing the flowability of cement.
In any case, in order to further improve cement of high strength and high flowability in strength, it is required to improve the flowability of the whole binder including cement, to further decrease the unit water content while maintaining the flowability of concrete, or to restrict the calorific value of hydration of the whole binder including cement to improve the strength development of the binder.
However, flowability and strength are essentially conflicting with each other, and there exists a relationship between these that if one is improved, the other is degraded. Therefore, it is not easy to solve these two subjects simultaneously. In addition, it is difficult to achieve an additional subject of decreasing the calorific value of hydration, and it had not been possible by the prior art to solve all of these subjects.
It should be noted that various types of concrete of high flowability and high strength are being used in these years, and while these kinds of concrete exhibit superior characteristics, they also present a drawback in that they increase the autogenious shrinkage (phenomenon in which a volumetric decrease occurs after initial setting of cement by the hydration reaction) thereof.
On the other hand, in case of a concrete member having a large sectional area, the heat of hydration of the cement member is accumulated in the proximity of its center to result in a rise of internal temperature. During its temperature rising process or cooling process, a considerable temperature difference occurs between the exterior (portions coming into contact with open air) and interior of the placed concrete and causes partial strains, whereby so-called thermal cracks are apt to occur.
In case such thermal cracks are likely to occur, it needs to be treated as mass concrete in terms of design and working.
While there are various methods of preventing thermal cracks in mass concrete, the use of cement of low calorific value is considered to be most effective and economical.
An example of such cement of low calorific value is blended cement in which ground granulated of blast-furnace slag and/or fly ash are admixed to portland cement by huge amounts. However, this blended cement presents a drawback in that the appearance of initial strength is small and thus results in delays in removal of forms or in inferiority in view of resistibility.
Low-heat portland cement is employed in order to solve these problems, but since such low-heat portland cement is still not capable of sufficiently decreasing the calorific value, it can not exhibit satisfactory effects in preventing thermal cracks.
The problem of autogenious shrinkage does also apply to mass concrete. That is, instances are reported in which cracks occurred which can not be simply explained by the reason accompanying the above described temperature differences, and it is pointed out that it is possible that autogenious shrinkage largely influences the occurrence of such cracks.
It is an object of the present invention to solve the problem of preventing heat of hydration simultaneously with improving the flowability of cement and maintaining a long-term strength thereof in order to further improve concrete of high strength and high flowability in strength.
It is another object of the present invention to provide cement for concrete of high strength and high flowability that exhibits high flowability and low viscosity at low water cement ratio, high strength development even after receiving thermal hysteresis, and a low amount of autogenious shrinkage.
It is still another object of the present invention to provide cement for mass concrete that exhibits superior effects of preventing cracks owing to thermal cracks and autogenious shrinkage.
In order to achieve these objects, the present invention is characterized in that the amount of Al2O3 and Fe2O3 contained in cement clinker are set to be 0.05 to 0.62 by Al2O3/Fe2O3 ratio.
It is known that 3CaO.Al2O3 (hereinafter referred to as xe2x80x9cC3Axe2x80x9d) in cement exhibits high hydration reactivity, high heat of hydration and large autogenious shrinkage amounts. Therefore, when manufacturing moderate heat portland cement or low-heat portland cement, the amount of contained C3A is decreased to approximately 2 to 4% by weight as compared with 9% by weight contained in normal portland cement.
The only interstitial material calculated by the Bogue calculation when A2O3/Fe2O3 ratio (hereinafter referred to as xe2x80x9cIMxe2x80x9d of the cement clinker is approximately 0.62 is ferrite (C4AF), and no C3A is calculated on estimatory basis.
By further decreasing IM, the interstitial material changes to ferrite of C6AF2 composition and further to that of C2F composition.
By omitting C3A which exhibits high hydration reactivity, high calorific value of hydration, high adsorption amount to chemical admixture when employed as concrete as well as largest amount of autogenious shrinkage, it is possible to improve the flowability and to decrease the amount of the autogenious shrinkage.
As a further advantage, accompanying the change of the ferrite composition from C4AF to C6AF2 by the decrease of IM, cement decreases in the heat of hydration and improves in the flowability thereof.
Since C4AF has an amount of autogenious shrinkage second as large as that of C3A, and the autogenious shrinkage of C6AF2 is relatively small, the amount of autogenious shrinkage is further decreased by the above changes in composition.
It has become apparent by the present invention that changes in ferrite composition by decreasing IM in the cement clinker is a more effective measure to solve the subjects than obtained by the effect of simply omitting C3A.
It should be noted the elongation of compressive strength of mortar is insufficient in case the interstitial material is ferrite of C2F composition (IM: in the proximity of 0), so that the lower limit value of IM has been set to 0.05. Possible reasons for this are influences on 2CaO.SiO2 (hereinafter referred to as xe2x80x9cC2Sxe2x80x9d) due to changes in solid solution rate of minor elements and also large sized crystals of C2S owing to a decrease in Al2O3 amount in the cement clinker whereby C2S is hard to be ground during the grinding process to cement.
In the present invention, it is preferable to set the amount of contained C2S to 35 to 75% by weight.
When the amount of C2S contained in the cement clinker is less than 35% by weight, the amount of contained 3CaO.SiO2 (hereinafter referred to as xe2x80x9cC3Sxe2x80x9d) relatively increases, whereby the heat of concrete increases, the strength development after receiving thermal hysteresis decreases, and the strength per amount of adiabatic temperature rise (strength per rise of adiabatic temperature by 1xc2x0 C.) of concrete does not become large than compared to that of low-heat portland cement.
On the other hand, when the amount of C2S exceeds 75% by weight, the strength development speed is delayed, and the specified strength can not be obtained. Further, the strength per amount of adiabatic temperature rise of concrete does not become larger than that of low-heat portland cement.
The strength per amount of adiabatic temperature rise for concrete is employed as an index of resistibility of concrete against thermal cracks.
The resistibility of concrete against thermal cracks becomes larger when the ratio of tensile strength to thermal stress of concrete becomes larger.
That is, the resistibility against thermal cracks becomes larger when the thermal stress becomes smaller, provided that the strength is identical, and when the tensile strength becomes larger, provided that the thermal stresses are identical.
When conditions for building are identical, the developmental stress becomes larger when the calorific value of concrete becomes larger.
While detailed evaluation of resistibility of concrete against thermal cracks needs to be performed by thermal stress analysis employing, for instance, the finite element method; but when comparing the resistibility of concrete itself against thermal cracks, the strength (compression) per rise of adiabatic temperature by 1xc2x0 C. is simply employed as an index.
The reason for this is that tensile strength and compressive strength are correlated to each other.
It is further possible in the present invention to include 2 to 25% by weight of fly ash of a particles size not more than 20 xcexcm in a cement compound containing a cement clinker in which the amount of Al2O3 and Fe2O3 are 0.05 to 0.62 in Al2O3/Fe2O3 ratio.
The reason why the particle size of fly ash, if ever included, should not be more than 20 xcexcm is that fly ash of a particle size of not more than 20 xcexcm can be filled between cement particles whereby the flowability improves without increasing the viscosity due to filler effect.
The reason for setting the admixing amount of fly ash to 2 to 25% by weight is that no sufficient filler effect can be obtained in case this mixing amount of fly ash is less than 2% by weight; on the other hand, the strength development speed is delayed so that no specified strength can be obtained in case this amount exceeds 25% by weight.
Further, it is also possible to admix, instead of fly ash, 10 to 60% by weight of ground granulated of blast-furnace slag whose specific surface area by blaine is 5,000 to 10,000 cm2/g.
When the specific surface area by blaine of the blast-furnace slag powder is not less than 5,000 cm2/g, the blast-furnace slag is filled between the cement particles whereby the flowability improves without increasing the viscosity due to the filler effect. On the other hand, when it exceeds 10,000 cm2/g, the amount of dispersing agent used for obtaining a specified workability of concrete is considerably increased and is thus uneconomical.
When the admixing amount of blast-furnace slag powder is less than 10% by weight, no sufficient filler effect can be obtained, and the strength development speed is delayed so that no specified strength can be obtained when this amount exceeds 60% by weight.
It is also possible to include dispersing agent in the present invention.
Dispersing agent is used for the purpose of securing the flowability by dispersing cement particles at low water cement ratios so as to decrease the yield value of the cement paste. The composition of the dispersing agent is not especially limited, provided that it is capable of dispersing cement particles, and any commercially available high-range water reducing agents or high-range AE water reducing agents may be employed.
When applying the cement of the present invention to concrete of high strength and high flowability, the admixing proportion of cement, water, aggregates and dispersing agents are not especially limited. However, it is most effective when the cement of the present invention is applied to concrete of high strength and high flowability having a water cement ratio not more than approximately 30% at which the viscosity of concrete remarkably increases.
As discussed so far, since the clinker composition has been set to be 0.05 to 0.62 by Al2O3/Fe2O3 ratio, the present invention has made it possible to solve the problem of preventing heat of hydration while improving the flowability of cement and maintaining a long-term strength thereof which the prior art was not capable of solving. Consequently, conventional problems of low-heat portland cement and problems which occurred when adding, for instance, fly ash can be solved, and there can be provided a clinker composition suitable for concrete of high strength and high flowability.
Further, there can be obtained concrete of high strength and high flowability which exhibits high flowability and low viscosity at low water cement ratio and which also exhibits high strength development even after receiving thermal hysteresis.
Accordingly, it becomes possible to work with concrete of high strength and high flowability with a specified concrete strength of approximately 80 N/mm2 without the limitations in types of pumps or pumping conditions. In addition, there are no restrictions for aggregates capable of being used. Whereby, concrete of high strength and high flowability can easily be manufactured.
The amount of autogenious shrinkage can also be decreased, with the result that the occurrence of cracks accompanying the autogenious shrinkage can be prevented, and the resistibility can be improved. Thus, functions of concrete of high strength and high flowability can be highly promoted.
Further, resistibility against thermal cracks in mass concrete structures is improved, with the result that it becomes possible to decrease the amount of any other measures for preventing thermal cracks which had been generally used in a combining manner. Whereby, thermal cracks can be decreased in an economical way.